This invention relates to an exhaust gas purifying apparatus for use with an internal combustion engine, especially for an automobile. There is known an exhaust gas re-circulating system which is adapted for reducing the amount of nitrogen oxides (NOx) by re-circulating part of exhaust gas from an engine into its inlet side. There is also another apparatus which purifies exhaust gas by introducing additional air into the outlet side of the engine and burning carbon monoxide (CO) and residual hydrocarbons contained in the exhaust gas. In a known automobile engine, these two systems are generally controlled independently of each other, causing complication of the control system and high production cost. Furthermore, use of these apparatus tends to increase fuel consumption.